The most generally used technology in the prior art for improving the workability of steel wire rod is the method of performing spheroidizing annealing. The prior art utilizing spheroidizing annealing, as shown in PLT 1, includes making the grain size of the austenite crystal 100 μm or more and making the volume fraction of ferrite 20% or less. In particular, as a method for promoting spheroidizing of cementite after annealing, Cr is added.
In this prior art, to secure forgeability, the grain size of the austenite crystal has to be made 100 μm or more, so when performing a forging operation in which a free surface is exposed and worked instead of performing an upset operation, the skin of the free surface part is caused to be uneven in shape. If the extent of this is severe, the result may become to be relatively noticeable unevenness like an orange peel. Depending on use for products, the unevenness may become a problem. Further, since a lot of Cr is added for improving the formation of cementite, the cost of the alloy steel also becomes somewhat higher and other problems are incurred.
PLT 2 controls the structure of a steel material so as to give degenerate pearlite: 10 area % or more, bainite: 75 area % or less, and ferrite: 60 area % or less, and achieves both shortening of the spheroidizing annealing time of the steel material and improvement of the workability and reduction of the deformation resistance after spheroidizing.
Further, PLT 2 restricts the area % of degenerate pearlite, bainite, and ferrite to desirable ranges to thereby achieve a balance of workability and deformation resistance and obtain a steel wire rod exhibiting excellent cold formability.
Further, PLT 3 describes a method for producing a rolled steel wire made of steel such as eutectoid steel. The method is characterized by producing a high tensile strength steel wire having excellent wire drawability by performing heat treatment for isothermal transformation immediately after completing the rolling without allowing the steel material to be transformed from the austenite phase in the integrated process from casting to wire rod rolling.
However, in PLTs 1 to 4, causes by which steel wire tends to be easily broken at the time of severely working steel wire rod to produce steel wire have not been researched. Further, the effects of the behavior of microvoids formed at the time of shaping steel wire rod into steel wire on the breakage of steel wire have not been researched.